Fish FAQ - Are sea slugs goby hosts?

John Weinberg sent the top image to Dr Bill Rudman at the Sea Slug Forum. The image shows the nudibranch, Ceratosoma trilobatum. John's sharp eyes saw not only the commensal shrimp, Periclimenes imperator, on the nudibranch but also the fish to the right of the shrimp.

Bill contacted the Australian Museum's fish Collection Manager, Mark McGrouther, who identified this tiny transparent fish as a juvenile goby. Mark forwarded the image to Australian Museum goby expert Dr Doug Hoese who confirmed that the fish was a goby and then sent the image to Dr Helen Larson who is an expert on this group of gobies.

Helen responded: "- that's neat! I've collected Pleurosicya off holothurians, seapens and Tridacna but never nudibranchs. The fish may be a Pleurosicya, yes, but it has very wide-set eyes like Bryaninops. Mind you it's just a child, so is not easy to identify."

Less than a fortnight later, Bill added another 'nudibranch and fish' image to the Sea Slug Forum. The bottom image on this page, which was taken by Jerry Allen, was also sent to Helen Larson for comment. Helen emailed back: “ – looks like a teeny, tiny baby Pleurosicya that has just settled out. Weird!”

Not enough is known about the association between these small fishes and their nudibranch hosts to comment on their type of symbiotic relationship.

Many fish species exhibit symbiosis at some stage of the life cycle. Fishes are known to live in three kinds of symbiotic relationships - parasitism, mutualism and commensalism.

Parasitism is a term used to describe a relationship in which one species benefits at the expense of another. It occurs in three families of fishes, the cutthroat eels, candiru catfishes and pearlfishes. They are all internal parasites of other animals that survive by eating the flesh or blood of the host species.

In commensalism one species benefits and the second is not harmed or helped. Many species of fishes have commensal relationships with invertebrates. Examples include fishes that shelter in between the branches of corals, in sponges or even inside various echinoderms, which are entered via the anus. Other commensal relationships include the shrimpfishes such as the Razorfish that shelters between the spines of sea urchins and the suckerfishes that often live with sharks and other large fishes.